Check if the file exists

Create the log.txt file in the same directory of this program to detect.

package main

import (
    "os"
    "fmt"
)

func main() {
    if _, err := os.Stat("log.txt"); err  == nil {
        fmt.Println("Log.txt file exists")
    }
}

Check if the file does not exist

package main

import (
    "os"
    "fmt"
)

func main() {
    // IsNotExist function definition: func IsNotExist (error) bool
    // Returns a boolean indicating whether the error indicates that a file or directory does not exist
    if _, err := os.Stat("log.txt"); os.IsNotExist(err) {
        fmt.Println("Log.txt file does not exist")
    } else {
        fmt.Println("Log.txt file exists")
    }
}

Read file contents

Create the name.txt file in the same directory of this program to detect.

package main

import (
    "io/ioutil"
    "fmt"
)

func main() {
    contentBytes, err := ioutil.ReadFile("name.txt")
    if err == nil {
        // ReadFile returned [] byte
        fmt.Println(string(contentBytes))
    }
}

write file

Create a hello world file in the same directory as this program.

package main

import (
    "io/ioutil"
    "fmt"
)

func main() {
    hello := "Hello, World"
    // WriteFile takes three parameters: the file name to write, the [] byte to write, and the write permission.
    err := ioutil.WriteFile("hello_world", []byte(hello), 0644)
    if err != nil {
        fmt.Println(err)
    }
}

Create temporary file

package main

import (
    "io/ioutil"
    "fmt"
)

func main() {
    helloWorld := "Hello, World"
    // TempFile definition: func TempFile (DIR, prefix string) (f * os.file, error error)
    // Create a new temporary file with prefix in dir directory.
    // Open the file in read-write mode and return the os.File pointer.
    file, err := ioutil.TempFile("", "hello_world_temp")
    if err != nil {
        panic(err)
    }
    // defer os.Remove(file.Name())
    if _, err := file.Write([]byte(helloWorld)); err != nil {
        panic(err)
    }
    fmt.Println(file.Name())
}

Calculate file lines

Create "names.txt" file in the same directory of the program, and write a few lines at random.

package main

import (
    "os"
    "bufio"
    "fmt"
)

func main() {
    file, _ := os.Open("names.txt")
    fileScanner := bufio.NewScanner(file)
    lineCount := 0
    for fileScanner.Scan() {
        lineCount++
    }
    defer file.Close()
    fmt.Println(lineCount)
}

Read file specific lines

package main

import (
    "os"
    "bufio"
    "fmt"
)

func main() {
    fmt.Println(readLine(2))
}

func readLine(lineNumber int) string {
    file, _ := os.Open("names.txt")
    fileScanner := bufio.NewScanner(file)
    lineCount := 0
    for fileScanner.Scan() {
        if lineCount == lineNumber {
            return fileScanner.Text()
        }
        lineCount++
    }
    defer file.Close()
    return ""
}

Compare the contents of two files

package main

import (
    "fmt"
    "io/ioutil"
    "bytes"
)

func main() {
    one, err := ioutil.ReadFile("one.txt")
    if err != nil {
        panic(err)
    }
    two, err2 := ioutil.ReadFile("two.txt")
    if err2 != nil {
        panic(err2)
    }
    // Equal returns the bool value
    same := bytes.Equal(one, two)
    fmt.Println(same)
}

Delete files

package main

import "os"

func main() {
    err := os.Remove("new.txt")
    if err != nil {
        panic(err)
    }
}

Copy or move files

package main

import (
    "os"
    "io"
)

func main() {
    original, err := os.Open("original.txt")
    if err != nil {
        panic(err)
    }
    defer original.Close()
    original_copy, err2 := os.Create("copy.txt")
    if err2 != nil {
        panic(err2)
    }
    defer original_copy.Close()
    // Copy (1, 2) - > copy 2 to 1
    _, err3 := io.Copy(original_copy, original)
    if err3 != nil {
        panic(err3)
    }
}
// Move file: use os.Remove() to delete the original file after copying

rename file

package main

import "os"

func main() {
    os.Rename("old.txt", "new.txt")
}

Delete directory and file

package main

import (
    "os"
    "fmt"
)

func main() {
    // Remove cannot delete a non empty folder. RemoveAll can
    err := os.RemoveAll("hello")
    if err != nil {
        panic(err)
    }
}

List all files in the directory

Create a hello folder. Create two files in the folder. The main file and the Hello folder are side by side.

.
├── hello
│   ├── 1.txt
│   └── 2.txt
└── main.go

package main

import (
    "io/ioutil"
    "fmt"
)

func main() {
    files, err := ioutil.ReadDir("hello")
    if err != nil {
        panic(err)
    }
    for _, f := range files {
        fmt.Println(f.Name())
    }
}